It was known that cell division took place only after periods of growth that have led to a multiplication of nuclei and in the tissues of plants above the thallophytes is very generally a part of the history of each mitosis.
This was thought because of the structure called the cell plate which was essentially an organ of cell division. But the thallophytes present other methods of cell division which bear no special relation to nuclear activities, and in certain groups of the thallophytes nuclear division may proceed through the entire vegetative life of the organism without any segmentation of the protoplasm which only takes place during the reproductive phase of spore formation.
But fundamentally protoplasmic segmentation depends on increase in the amount of protoplasm which demands the multiplication of nuclei so that nuclear division always precedes cell division, and we shall consider the events in that order.
Stages of Plant Cell Division
Events of Plant Nuclear Division
1. Direct Division.
The nucleus divides after one or two methods, either directly by constriction or fragmentation, or indirectly (mitosis) when there is present a fibrillar apparatus called the spindle. Direct division is the only form present in the simplest plants and phy logenetically must have preceded the elaborate mechanism de manded for indirect division. This topic will be given especial attention in Section VI '. Direct division is also present in cer tain specialized cells and tissues of higher plants. These are generally old cells or tissues that are far removed from the gen eralized structure and potentialities of germ plasm. Yet some times direct and indirect division occur in the same cell, e. g., Valonia (Fairchild, '94), and such forms might be made the subject of very interesting investigations. In some cases the phenomenon ,of direct nuclear division accompanies pathological conditions or the degeneration of cells and may take the form of extensive fragmentation. It would be outside of our purpose to discuss such phenomena which is obviously abnormal, and the primitive forms of nuclear division will be taken up later (Sec tion VI). It is possible that direct division in higher plants is in a sense a reversion to early ancestral conditions, a reversion that only comes on when for some reason the normal activities of the germ cell are in abeyance or have ceased.
2. Indirect Division (Mitosis).
Indirect nuclear division, mitosis or karyokinesis, is character ized by a mechanism which varies greatly among plants in its method of development. The characteristic appearance of this apparatus is a spindle like figure formed of fibrillae. The poles of the spindle may be occupied by centrosomes or centrospheres or they may be entirely free from such organized kinoplasmic bodies. The essential structures of the spindle are sets of con tracting fibers which separate the chromosomes into two groups drawing them to the poles of the spindle where the daughter nuclei are organized. But besides these fibers there are gen erally present other fibrillae which complicate the nuclear figure. Some of these extend from pole to pole (spindle fibers) others lie outside of the spindle and end freely in the cytoplasm or attach themselves to chromosomes (mantle fibers), and if centro somes or centrospheres be present there are likely to be fibers radiating from these centers to form asters.
The events of Plant mitosis are generally grouped into four periods :
(a) Prophase, to include the formation of the spindle and preparation of the chromosomes ;
(b) Metaphase, the separation of the daughter chromosomes ;
(c) Anaphase, the gathering of the daughter chromosomes into two groups which pass to the poles of the spindle ;
(d) Telophase, the organization of the daughter nuclei. It is almost needless to say that these periods merge so gradually one into the other that sharp lines cannot be drawn between them. The activities during prophase are especially variable.
